EUCAST is increasingly becoming the dominant international standard for various reasons and its procedures and documentation, including breakpoint tables are available online for free. For a recent overview of EUCAST, see this Journal of Antimicrobial Chemotherapy 2014 lecture (free access).

EUCAST-Expert-rules-version 2- essential reference for intrinsic resistances in Gram positive and negative bacteria. Where a species is intrinsically resistant to an antibiotic, there is no rationale for testing susceptibility